1. Field of the Invention
The present invention relates to an information processing apparatus including a recording means and a reproducing means having both high-density recording capacity and erasing functionality. The present invention also relates to an information processing method including recording and erasing, and further to a recording medium employed therefor.
More particularly, the present invention relates to a recording-reproducing apparatus and a recording-reproducing-erasing method by which recording is conducted by accumulating an electric charge on a photoconductive layer, reproduction of the recorded information is conducted by detection of the electric charge quantity as an electric current or a voltage with a probe electrode, and erasure of the information is conducted by discharging the accumulated electric charge by light projection.
2. Related Background Art
In recent years, memory elements and memory systems are utilized in a variety of application fields such as computers and related apparatuses, video discs, digital audio discs, and so forth, and are the key materials in electronics industries. Generally, memory systems are required to have the performances below:
(1) large capacity in small volume,
(2) quick response in recording and reproduction,
(3) low error rate,
(4) low power consumption,
(5) high productivity and low cost,
and so forth. Heretofore, magnetic memory devices and semiconductor memory devices have been principally used for the memory systems. As the results of the recent progress of laser technique, there have come to be used optical memory devices and the like employing an inexpensive and high-density recording medium. However, the memory devices or methods are desired practically which have still larger capacity and smaller volume to adapt to the family use of computors and increase of image information.
On the other hand, scanning tunnel microscopy (hereinafter referred to as "STM") has been developed which enables direct observation of the electronic structure of an atom on a surface of a conductor (G. Binnig et al., Phys. Rev. Lett. 49 (1982) 57). By the STM, a real spatial image of an amorphous substance as well as a single crystal has become measurable with remarkably high resolution (at a nanometer level or less). The STM utilizes the tunnel current which flows through a metallic probe when the metallic probe is brought close to an electrocondutive substance at a distance of about 1 nm. This current is extremely sensitive to the change of the above distance, so that the surface structure of a real spatial image can be traced by scanning with a probe so as to maintain the tunnel current constant. The analysis by STM has been applicable only to electroconductive materials. However, the STM has begun to be utilized for analysis of the structure of a thin insulating film formed on a surface of an electroconductive material. Since the apparatus and the means for the STM are dependent on detection of a minute current, the observation can advantageously be conducted without impairing the medium and with low electric power. Furthermore, the operation of STM may be conducted in atmospheric environment.
Accordingly, the STM is promising in a broad range of applications. In particular, use as a reproduction apparatus is actively studied for reading out information written in a sample with high resolution. For example, Quate, et al. of Stanford University disclosed a method of injecting an electric charge to an insulating layer interface by applying a voltage with an STM probe (for recording) and detecting the charge by a tunnel current flowing the probe (for reproduction); (C. F. Quate, U.S. Pat. No. 4,575,822). By utilizing such a method, a memory apparatus of extremely high density can readily be realized.
The process is specifically conducted by the procedure mentioned below by referring to FIG. 6 as an example. A recording medium suitable for the process comprises an electroconductive silicon semiconductor substrate 1 doped with an impurity, a silicon oxide film layer 2 formed on the substrate 1, and a silicon nitride film layer 3 formed on the silicon oxide film layer 2. A probe electrode 5 is brought into contact with the silicon nitride film layer. A predetermined voltage is applied to the insulating layer by applying a voltage between the probe and the substrate by means of a voltage applying apparatus 4. Consequently, electrons tunnel through the insulating layer, and an electric charge accumulates at the interface in the insulating layer. In FIG. 6, the symbols a, b, c, and d respectively denote regions of first data, second data, third data and fourth data. The numerals 6 and 7 denote the charge (or data) at the positions a and c, respectively. The probe is then removed from the insulating layer surface, leaving the electron having tunneled in a trapped state. In reading out the information recorded by the trapped electrons, the probe is brought sufficiently close to the silicon nitride film layer and the trapped electrons, and a voltage is applied between the substrate and the probe so as to produce a tunnel current, and simultaneously the distance of the probe from the insulating surface is varied.
Usually, the probe bias for reading out the data is of opposite polarity to the probe bias for the data recording. The measured tunnel current denotes the presence or absence of an accumulated charge at the interface in the insulating layer interface. The region for accumulation of 1 bit of data is as small as an order of 10.sup.-4 .mu.m.sup.2 of the surface area. Consequently, a memory device having a large capacity, for example, of 100M byte can be made in a level of as small as 1 cm.sup.3 in volume.
Quate et al. showed in the aforementioned patent specification that information recorded by "disorder" (e.g., physical roughness, variation of an electric state, etc.) formed by physical probing, a focused laser beam, an electron beam, adhesion of fine particles, or the like on the surface of a recording medium as well as the charge accumulation can be readily be read out by utilizing the tunnel current. Practically, the recording by charge accumulation is promising in view of the density, reproducibility, and readiness of the recording.
The above-mentioned recording-reproducing method, however, involves a disadvantage that the information once recorded is not readily erasable. In an erasing step in the above method, access with a probe is required to every one bit just like in the recording step. Therefore, the time for erasure increases in proportion to the amount of erasure, and rapid response is not readily attained for a broad range of erasure. Further, a bias voltage of polarity opposite to the accumulated charge have to be applied. Under such circumstances, high controllability is required for the voltage, the distance between the medium and the probe, and so forth. Deviation from the optimum conditions may cause insufficiency of erasure, or further accumulation of an opposite polarity charge. Accordingly, an erasure mechanism or procedure is desired which is accessible to many bits at a time without use of a probe.